1. Field of the Invention
The present invention relates to a voltage regulator for alternator and more specifically, to a short circuit protection device for use in an alternator for short circuit protection.
2. Description of the Related Art
In a regular vehicle alternator 1, the field coil 10 of the rotor has its two opposite ends in conduction with the DC power source 30 of the storage battery and the voltage regulator 20 through the slip ring and the brush. When the internal circuit of the voltage regulator 20 is normal, the field coil 10 provides a field current that causes the stator to induce a magnetic field upon rotation of the rotor. The alternative current produced upon induction of the stator coil 11 is rectified by the rectifier 12 into direct current for charging the storage battery and functioning of the vehicle electric devices. Therefore, during normal running of the alternator 1, the voltage regulator 20 monitors the potential of the storage battery, and regulates the field coil 10 of the rotor, and controls the stator coil 11 and the rectifier 12 to charge the storage battery. Therefore, the storage battery has sufficient electric capacity to support normal operation of the electric devices of the vehicle.
Referring to FIG. 2, the DC power source 30 of the storage battery goes through the input end of the power transistor 21 of the voltage regulator 20 to conduct the power transistor 21, causing the field current of the field coil 10 to charge the storage battery. When the electric capacity of the storage battery is charged to the saturated potential, the step voltage produced at the zener diode 22 reaches the breakdown critical voltage to conduct the posterior switching transistor 23, and to draw down the control base of the power transistor 21 to low potential, thereby shutting off the power transistor 21 and stopping charging of the storage battery. When the electric capacity of the storage battery is consumed to the potential that is unable to conduct the zener diode 22, the power transistor 21 is conducted to charge the storage battery again. Therefore, the voltage regulator 20 prohibits overcharge of the storage battery by the alternator, maintaining normal operation of the storage battery.
However, vibration of the body of the vehicle after start of the vehicle and continuous rotation of the rotor cause the external insulation of the enameled wire of the rotor to wear quickly. When the brush starts to wear, the fallen graphite dust, fallen solder dust, or any fallen conducting substance may cause a partial short circuit or full short circuit of the field coil 10. At this time, the field current of the short circuit may cause the voltage regulator 20 to enhance the power generating operation of the alternator, and rising of the potential of the storage battery may cause the zener diode 22 to be conducted again so as to turn off the short-circuit current. However, when the potential of the storage battery dropped to the conduction level of the power transistor 21, the short circuit current will go through the power transistor 21. Therefore, the power transistor 21 is being frequently switched between the conduction status and the shutoff status to cause a short circuit oscillation of the voltage regulator 20. The internal junction of the power transistor 21 may be unable to sustain this surge short-circuit current, causing the power transistor 21 to burn out. This short circuit oscillation may cause an overcurrent at the field coil 10, and this overcurrent may cause the power transistor 21 to burn out. When the power transistor 21 is damaged, the alternator may be forced to provide an overvoltage to the electric devices of the vehicle, causing damage to the electric devices.